Method for providing oxide coating on aluminum and its alloys



Patented Dec. 11, 1951 MET-HGD FOR PROVIDING OXIDE COATING ON 'ALUMIN UM AND" IT S ALLOYS Charles. C. Qohn, Elkinsfarklfm.

No Drawing-.. Application March 29', 1947; SerialNo. 738,220

2. Claims. CL ZMr-SS') This: invention relates to the treatment of. aluminum or aluminum-alloysurfaces to provide aluminum oxide.- coatings: thereon f or; the purpose of increasing resistance to corrosion, of insulation, of; dyeing;v on for other; usual purposes of suchtype of: treatment.

This application is in, part. a: continuation of my appli'cation; Serial N02, 543,046, filed June 30, 194.4,now abandoned.

Many methods have: been proposed for. the treatment oil aluminum and its alloys, involving generally the-Iormationonthe aluminum'or alloy surface of a hard. film. of 1 oxide which; isresistant to abnasionandalsoserves to p1ievent,,to a. rea: sonab1e.cleg1;ee,-, chemical corrosionof the. aluminum material.

Thebest results in: the direction of. providing,

produced by various other methods due to its nonporous nature, and in spite of its thinness. On the otherhand, ithas relativelylow abrasion resistance-and in view of its lack of porosity it results in relatively poordyeing. This anodized coat produced by the use of chr-omic acid is accordingly used in practice mainly for corrosion resistance orasa paint base. Similar thin, impervicus coatings may be producedbyother treatments-as described hereafter:

Incontrast with the above, the oxide coats formed by anodizing usingsulphuric acid or oxalic acidelectrolytes are thicker and moreahrasion resistant but are substantially more porous. It is customary-andgenerally necessary to seal these pores by plugging them withwaxes or oils or by thechemical deposition. of insolublemetallic compounds by immersion of the coat inahot solution of some-weakmetallic base-or in a hot Solutionofanalkali bichromate to precipitate an insoluble chromate;

of sealing have various disadvantages. Discolor ationof the oxide coat generally takes place to an appreciableextentxand in the case of chromate sealing to avery marked extent. Alongimmers-iom; period in. a hotsolutionismequiredr While:

These various methods 2-. the corrosion. resistance is increased. the ultimate resistance securedis usually not as good asthat attainable by'ohromicacid anodizing.

In accordance with the present invention the aluminum. or, aluminum alloy issubjected to an anodizing treatmentot conventional type using as the-electrolyte a sulphuric or. oxalic acidsolu tion or one of the various. other known. electrolyteswhichgive rise-tuna. porous oxide coat. This conventional. oxide-forming. treatment is not, alone,v a part, of the present inventionandneed notv be further. specifically described; except. to note that. it. may equally well be carried. out through the. use. of. either. direct or alternating,

current. in accordance. with the usual. practices in.

this. art. Slfl-llhurio-chromic acid, mixtures may also housed. at this stage sincethey will give rise to porous, thickcoats... To. produce these porous coats there may alsobe used. such electrolytes as phosphoric acid, malonic acid, hydrofluosilicic acid, manganic acid, various sulphonic and sulphamic acids, potassium bisulphate and others known to the art. The electrolytes may contain metallic salts or other constituents as are known in the art and may be used under conventional conditions of times, temperatures, current densities, voltages, concentrations; etc.

As an example of the anodic treatment just described there may be cited anodic treatment in a conventional sulphuric acid electrolyte comprising, for example, a 10% sulphuric acid solution, the anodictreatment being carried out for twenty minutes at fifteen volts at temperatures rangingfrom' 76 F: to 95 F; Wide variations in thesevalues are permissible as will be evident by reference toprior patents and publications in this art.-

Following theformation of a porous coat by the conventional anodic treatment indicated above, which porous coat is generally desirably thick, though it may be thin in particular cases, the aluminum or aluminum alloy surfaces which have been coated are subjected, in accordance with the invention, to anodic electrochemical action in a solution which, preferably, may beone which gives rise to formation of chromic acid at the anode. This anodic treatment, for example, may becarried' out in a 1% to50% aqueous solution of chromium trioxide, preferably about 10% (or in asolution of equivalent anion content) for one to three minutes'at twenty to fifty volts and at a temperature ranging from room temperature. to about-130 F. This forms a thin imperviouscoatingwhich, alternatively, may be formed by; other: methods, known to. the art, giving rise to similar thin, impervious coatings, for example, using sodium silicate, boric acid, borates such as ammonium borate or borax, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, acetic acid and other known materials. The electrolyte should be free of substantial amounts of free sulphuric acid which would produce porous coatings.

While anodic after treatment has been referred to, it is not intended to imply that this can be carried out solely by the use of direct current. It has been found possible to use unrectified alternating current under approximately corresponding conditions of voltage, time and temperature, those half cycles being effective which correspond to an anodic condition of the material being treated. In fact, with the use of alternating current it is possible to treat two articles or groups of articles simultaneously, these articles or groups forming the two electrodes in the treating bath. As each article or group alternately becomes the anode its treatment occurs. It will, of course, be obvious that pulsating direct current as well as substantially constant direct current may be used.

A study of the procedure just described indicates what happens to render it highly effective. The originally formed coating (produced in a sulphuric acid, oxalic acid or equivalent electrolyte) is porous so that in the second chromicion containing, or equivalent, bath the electrolyte may readily penetrate the pores and the current reaches the metal surface through these pores. Accordingly, the further coating of oxide produced in the second treatment builds up directly on the aluminum or alloy surface with the result that the coating finally produced consists of a thin non-porous layer directly in contact with the metal surface, covered by the relatively porous, and generally relatively thick, coating produced by the first treatment which has now been pushed away from the metal surface. It is to be stressed that this second treatment does not involve sealing in the usual fashion characterized by the mere plugging of the pores of the porous coat. Rather the effect is to produce two distinct oxide layers of different characteristics, the inner involving a high degree of corrosion resistance and the outer a high degree of abrasion resistance and with other advantages attendant upon thickness and porosity.

That the outer layer remains porous, and is not merely plugged, is quite evident from the fact that it has substantially the dye absorbing properties characteristic of a coating produced by the first type of treatment (sulphuric or oxalic acid or the like) alone. The porous coating absorbs dye uninhibited by the relatively impervious coating which lies between it and the surface of the metal. While this relatively impervious coating does not absorb a substantial quantity of dye, that condition is not detrimental.

The process is, in fact, particularly advantageous when a dyed coating is to be produced having a high resistance to corrosion and also having high electrical resistance or abrasion resistance qualities. Dyeing may be carried out in any of the usual fashions well known in the art. Then the pores of the porous coating may be sealed in conventional fashion using, for example, nickel acetate, sodium aluminate, alkali silicates, or the like. While the underlying impervious coat takes care of corrosion resistance to a much greater extent than the sealing material, the seal is desirable to insure that the dye remains in the coat and cannot be washed out if the surface becomes wet. In fact, even without dyeing such sealing may be used to add somewhat to the corrosion resistance. When dyeing is not effected and coloration is not objectionable this sealing may be performed through the use of an alkali bichromate in the usual fashion.

The invention, as will be clear from the above, involves, fundamentally, first the formation of a porous coating followed by the formation of an impervious thin coating which underlies the porous coating, being formed through the pores thereof on the metal surface. It may be noted that a reversal of the order of formation of these coatings is quite ineffective: if the impervious coating is first formed, not only is a pervious coating formed with difficulty but, if formed (by prolonged treatment), the impervious coating is destroyed from the standpoint of effectiveness by being pushed away from the metal surface. At best, the result is about equivalent to that resulting from the formation of only a pervious coating, with inferior dye absorption characteristics.

One exception to the last exists, however: porous castings of aluminum or aluminum alloys may be advantageously subjected to a triple treatment, first involving formation of an impervious film, secondly formation of a porous film and thirdly formation of a final impervious film. The first is a pretreatment which may be carried out either chemically or anodically.

In accordance with the chemical method, a porous casting which has been previously cleaned is immersed for a period of about 5 to 40 minutes, and preferably about 30 minutes, in a hot solution containing about 1% to 7% by weight of a bichromate such as sodium, potassium or ammonium bichromate. The conditions of this treatment are not particularly critical. As to the time, 5 minutes treatment is apparently about an effective minimum while 40 minutes results in a condition which is not susceptible to substantial improvement by longer treatment. While the desired action takes place at lower temperatures, it is substantially accelerated by using temperatures of about 150 F. to boiling of the solution. The content of chromate or bichromate (the essential of the solution being solely that it contains chromic acid anions) may be varied through a considerable range, even beyond the limits just indicated, but it appears that about 5% by weight of sodium bichromate is optimum.

Even more desirable, however, is a pre-treatment giving rise to very similar results comprising the anodic treatment of porous castings of the type indicated, this treatment being carried out in a solution containing chromic acid anions which may comprise either a solution of chromic acid or solutions of chromates or bichromates which will give rise in electrolytic treatment to the formation of chromic acid at the anode. The solution in which treatment is effected is preferably about a 1% to 50% aqueous solution of chromium trioxide (1.35 ounces to ounces chromium trioxide per gallon of water) or a solution of chromate or bichromate having the equivalent chromic acid content. A 10% solution of chromium trioxide appears to give the best results, but this is not critical. The treatment is carried out with the material to be treated forming the anode and with an applied voltage of 20 to 50 volts, preferably about 40 volts, at a temperature which may range from ordinary room temperature to about 130 F., though the temperature limits are not critical. The time of such anodic treatment may be quite short, and, in fact, 1 to 3 minutes will generally sufiice to give substantially as good results as may be accomplished by much longer treatment.

This pre-treatment has the effect of producing a coating deep in the pores of the casting. This is then followed by formation of a porous film as described above which, while it does not penetrate the pores and does not destroy or substantially modify the coating therein, in effect lifts the surface film resulting from the pretreatment and destroys its impervious nature. Then the third treatment effects the same results as the last treatment above described for aluminum and its alloys generally. The second and third treatments are identical with the two earlier mentioned and the final results are quite similar, the only overall difference being more effective plugging of the casting pores.

Where aluminum is hereafter referred to it will be understood to include aluminum and alloys containing as their major constituent aluminum on which oxide coatings may be formed in accordance with the prior art procedures above referred to.

What I claim and desire to protect by Letters Patent is:

1. The method of treating aluminum comprising essentially the steps of successively forming anodically thereon a relatively pervious aluminum oxide coating and then anodically, in an electrolyte containing a substantial quantity of anions of chromic acid and substantially free of free sulphuric acid, a relatively impervious aluminum oxide coating underlying the relatively pervious coating.

2. The method of treating an aluminum casting comprising essentially the steps of successively pretreating the casting with a solution containing a substantial quantity of anions of chromic acid, forming anodically on the casting a relatively pervious aluminum oxide coating, and then anodically a relatively impervious aluminum oxide coating underlying the relatively pervious coating.

CHARLES C. COHN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,946,151 Edwards Feb. 6, 1934 1,946,152 Edwards Feb. 6, 1934 1,946,153 Edwards Feb. 6, 1934 1,952,339 Cotton June 12, 1934 1,965,683 Work July 10, 1934 1,977,622 Buzzard -1 Oct. 23, 1934 2,126,017 Jenny Aug. 9, 1938 2,448,513 Brennan et al Sept. 7, 1948 FOREIGN PATENTS Number Country Date 290,901 Great Britain May 24, 1928 390,110 Great Britain Mar. 30, 1930 453,226 Great Britain Sept. 1, 1936 OTHER REFERENCES Rogers: Manual of Industrial Chemistry, vol. I (1926), page 230.

Burns and Schuh: Protective Coatings for Metals (1939) pages 370, 371. 

1. THE METHOD OF TREATING ALUMINUM COMPRISING ESSENTIALLY THE STEPS OF SUCCESSIVELY FORMING ANODICALLY THEREON A RELATIVELY PERVIOUS ALUMINIUM OXIDE COATING AND THEN ANODICALLY, IN AN ELECTROLYTE CONTAINING A SUBSTANTIAL QUANTITY OF ANIONS OF CHROIC ACID AND SUBSTANTIALLY FREE OF FREE SULPHURIC ACID, A RELATIVELY IMPERVIOUS ALUMINUM OXIDE COATING UNDERLYING THE RELATIVELY PERVIOUS COATING. 